In electrophotography, generally, a surface of a latent image carrier is charged using corona discharge etc. followed by exposure using laser etc. to form an electrostatic latent image, which is then developed and visualized using a developer such as a toner to obtain an image with high quality. The toner used for such a development method is typically one produced by mixing a binder resin with components such as a colorant, a charge control agent, and a release agent to obtain a mixture, then which is further melted and kneaded, followed by pulverizing and classifying the melt-kneaded material to form toner particles with an average particle diameter of 5 μm or more and 10 μm or less. Then, in order to provide flowability to the toner, to perform charge control of the toner, and to facilitate cleaning of the toner from the surface of the latent image bearing member, typically, an inorganic fine powder such as of silica and titanium oxide is added to a surface of the toner. In regards to such a toner, typically, a nearly spherical toner with a high circularity is often used in order to improve flowability thereof.
In the electrophotography, a transfer residual toner remains on the latent image bearing member after toner images are transferred from the latent image bearing member. Such a transfer residual toner is typically removed from the surface of the latent image bearing member by a cleaning unit having a mechanism such as an elastic blade. However, in a case in which the toner has a higher circularity, the transfer residual toner may pass through the cleaning unit and remain on the latent image bearing member. In such a case, image defects due to the transfer residual toner may occur in resulting images.
For the countermeasure, in order to prevent the transfer residual toner from passing through when cleaning the transfer residual toner, for example, there have been proposed a toner having a plurality of concave portions on a surface of toner particles and being produced by a suspension polymerization process and a toner in which concave and convex portions are formed on a surface of toner particles such that spans between tops of convex portions are within a certain range.
However, the two types of the toners described above, which have been proposed to prevent the transfer residual toner from passing through when cleaning the transfer residual toner, tend to adhere to the surface of the latent image bearing member; therefore, image defects called “void” may occur in resulting images since a portion of toner images is not transferred during the transfer. Furthermore, in the two types of the toners described above, the toners may become resistant to being charged to a desired potential and thus image density of resulting images may become lower than a desired value when printing at a lower coverage rate for a long period and thus the toners receive stress for a long period due to stirring within developing units.
The present disclosure has been made in view of the problems described above; and it is an object of the present disclosure to provide a toner for electrostatic latent image development in which occurrence of image defects in resulting images due to the toner passing-through cleaning units and image defects in resulting images such as void can be suppressed and image density of resulting images does not become lower than a desired value even when printing at a lower coverage rate for a long period. It is a further object of the present disclosure to provide a method of producing a toner for electrostatic latent image development which is adapted to the method of producing the toner for electrostatic latent image development described above.
The first aspect of the present disclosure is a toner for electrostatic latent image development. The toner includes at least a colorant, a charge control agent, and a release agent in a binder resin and is produced by a pulverizing process. An average circularity of the toner particles is 0.960 or more and 0.980 or less with respect to toner particles having a primary particle diameter of 3 μm or more and 10 μm or less. Furthermore, when 100 by number of the toner particles thereof are observed using a scanning electron microscope, a numerical proportion of toner particles having a concave portion of which outer diameter, measured using an image of the scanning electron microscope, is 200 nm or more is 10% by number or less.
Another aspect of the present disclosure is a method of producing the toner for electrostatic latent image development which includes the following steps (i) to (v):
(i) a step of mixing a binder resin, a colorant, a charge control agent, and a release agent, followed by melting and kneading them,
(ii) a step of roughly pulverizing the melt-kneaded material resulting from the step (i) to obtain a coarsely pulverized material,
(iii) a step of finely pulverizing the coarsely pulverized material by dividing a fine pulverization of the coarsely pulverized material into a plurality of times in series to obtain finely pulverized material,
(iv) a step of classifying after the fine pulverization to obtain a classified material, and
(v) a step of heat-treating the classified material to obtain a toner with a predetermined volume average particle diameter.